anyone setup their LCD to auto-camera input in reverse?

basically
I want the caputer screen to switch to the backup camera when I shift into reverse when backing up.
Liek the new BMW's do on their factory navigation

I know I can tap the backup signal from the tranny
(same signal that turns on the white reverse lights)

but how will I get the screen to auto switch?
my lcd does have a seperate line for video input; besides the VGA input. If I can use this somehow, then it doesn't matter if the carputer is on or off then.
Or I can wire the signal to the carputer full time, and just have it display when I shift to reverse
sorry it's late, tough putting the idea into words